1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method which prints an image on a print medium by ejecting ink onto the print medium and more particularly to a method of controlling voltage pulses applied to electrothermal transducers (heaters) for ejecting ink.
2. Description of the Related Art
The ink jet printing apparatus forms an image by ejecting ink from print elements in response to an image signal to print a plurality of dots on a print medium. Such an ink jet printing system has many advantages over other printing systems, including high speed, high density printing, a color printing capability with a simple construction and a quietness during printing.
A construction that ejects ink from print elements has already been proposed and implemented in some types of printing apparatus, of which a type that uses electrothermal transducers (heaters) in print elements can eject small drops of ink at a high density and at a high frequency and thus has found a wide range of applications. An ink jet print head of this construction has a plurality of print elements arrayed at a density corresponding to a print resolution. Each of the print elements is provided with a liquid path to introduce ink to a nozzle opening and also an electrothermal transducer (heater) in contact with the ink in the liquid path. In ejecting ink from the print elements in response to an image signal, individual heaters are applied a predetermined voltage pulse to be energized to heat the ink. A rapid heating causes the ink in contact with the heater surface to produce a film boiling, in which an expanding bubble expels a predetermined volume of ink from the nozzle opening which flies and lands on a print medium forming a dot.
In the ink jet print head of the above construction, a volume of ink droplet ejected from individual print element (hereinafter referred to as an ejection volume) depends on a resistance of the heater installed in each print element. This is because the amount of heat produced by the heater to generate a bubble during the film boiling varies depending on the resistance of the heater. So, if, when a color image is printed by a plurality of print heads, there are variations in heater resistance among individual print heads for example, the ejection volume will differ from one print head to another, giving rise to a possibility of the image being printed showing different colors from desired ones.
Further, the ejection volume is influenced by the temperature of the print head or more directly by the temperature of ink near the heater. This is because an ink viscosity changes with an ink temperature and a volume of a bubble and its growth speed during the film boiling depend on the ink viscosity. For example, when the temperature of the print head is low, the ink viscosity increases, making a bubble volume small, with the result that the volume of ink ejected and therefore an area of a printed dot become small. Conversely, when the print head temperature is high, the ink viscosity lowers, making the bubble volume large, with the result that the volume of ink ejected and therefore the printed dot area increase. That is, even if the printing is done based on the same image data, an unstable print head temperature would make the size of dots formed on a print medium unstable, which in turn leads to unstable image density.
Further, when a color image is printed using a plurality of print heads, temperature variations among the different color print heads will likely result in a color produced differing from a desired one. Furthermore, if the temperatures of individual print heads change, the color produced will deviate unstably from target color coordinates
In the print head manufacturing process, the print heads with a bubble forming heater inevitably have some variations in heater resistance. Considering the print head construction, it is also inevitable that the temperature varies among the print heads depending on the environment in which the printing apparatus is used or the frequencies of use of individual color heads. However, in the ink jet printing apparatus variations in image density and color produced are not desirable. It is therefore one of important tasks with the ink jet printing apparatus to stabilize the ejection volume of the print heads.
Japanese Patent Laid-Open No. 5-031905 (1993) discloses a technology which applies two voltage pulses for each ink ejection and controls a pulse width stepwise according to the temperature of the print head to stabilize the ejection volume of ink. This ejection volume control is referred to as a double pulse drive control.
FIG. 1 is a timing chart showing the double pulse drive control. An abscissa represents time and an ordinate represents a voltage applied to the heater. One ejection is done by two pulses shown in the figure. A control circuit in the ink jet printing apparatus sets a pulse width of a pulse signal shown in the figure according to the temperature to stabilize a volume of ejected ink droplets. In the figure, P1 represents a preheat pulse application time, P3 a main heat pulse application time, and P2 an interval between the preheat pulse and the main heat pulse.
The preheat pulse is applied to warm ink near the heater surface and its application time P1 is set so as to keep the energy applied at a level that will not result in generation of a bubble. The main heat pulse on the other hand is applied to cause a film boiling in the ink warmed by the preheat pulse and thereby execute an ejection. Its application time P3 is set larger than P1 so as to produce an enough energy to generate a bubble.
As described above, the ink ejection volume is considered as being dependent on a temperature distribution of ink near the heater. Japanese Patent Laid-Open No. 5-031905 (1993) discloses a method which adjusts the pulse width P1 of the preheat pulse according to the detected temperature to realize a stable ejection volume. More specifically, as the detected temperature gradually increases, for example, the necessity of heating the ink near the heater surface decreases progressively. The preheat pulse width P1 is therefore set to decrease progressively. Conversely, when the detected temperature gradually lowers, the necessity of warming the ink near the heater surface progressively increases and the preheat pulse width P1 is set to increase progressively.
Japanese Patent Laid-Open No. 5-031905 (1993) discloses a construction in which a table having predefined P1 related to the detected temperature is stored in memory in advance. Further, this cited document also discloses a method which classifies the print heads into a plurality of ranks according to the ejection volume (heater resistance) under the same condition and which provides tables that match the plurality of ranks. The use of the double pulse drive control described in Japanese Patent Laid-Open No. 5-031905 (1993) makes it possible to maintain the ejection volume at a fixed level stably for all colors even if the heater resistance and temperature differ from one print head to another.
In the conventional double pulse drive control such as disclosed in Japanese Patent Laid-Open No. 5-031905 (1993), an energy applied to the heater is adjusted by changing the pulse width while keeping the drive voltage constant. It should be noted, however, that the stabilization of ejection volume can also be achieved with a single pulse by changing the pulse voltage and the pulse width simultaneously. Such ejection volume control methods (hereinafter referred to as single pulse drive controls) are disclosed in Japanese Patent Laid-Open Nos. 2001-180015 and 2004-001435.
In the ink jet printing apparatus with a heater, there is a tendency that the ejection volume is larger when a lower voltage pulse is applied for a longer duration than when a higher voltage pulse is applied for a shorter duration. This is considered due to the fact that the application of a lower voltage pulse for a longer duration causes an ink area that is heated up to a bubble forming temperature to spread more widely by heat conduction, whereas applying a high voltage rapidly heats only an area very close to the heater, causing an instant generation of a bubble, resulting in a smaller ejection volume. Japanese Patent Laid-Open Nos. 2001-180015 and 2004-001435 describe an ejection control method that takes advantage of such an ejection characteristic and which, when one wishes to increase the ejection volume, reduces the drive voltage and widens (elongates) the pulse width and, when one wishes to reduce the ejection volume, raises the drive voltage and narrows (shortens) the pulse width.
As described above, the ink jet printing apparatus of recent years seek to keep the ejection volume as stable as possible by adopting the double pulse drive control method described in Japanese Patent Laid-Open No. 5-031905 (1993) and the single pulse drive control method disclosed in Japanese Patent Laid-Open Nos. 2001-180015 and 2004-001435.
Comparison between the double pulse ejection volume control and the single pulse ejection volume control shows that the double pulse drive control that adjusts the preheat pulse application time at a relatively low voltage generally has higher control reliability. However, as ink droplets are becoming smaller and smaller in recent years, it is increasingly difficult to maintain small ejection volumes stably with only the double pulse ejection volume control. For example, when the print head temperature continues to rise after continuous printing operations, the width of the preheat pulse is narrowed to reduce the ejection volume. There are, however, cases where even after the pulse width has become zero, the ejection volume remains too large.
In such cases, the target ejection volume can be maintained by switching from the double pulse drive control to the single pulse drive control when the preheat pulse width becomes zero. Then, small droplets of a predetermined volume can be expected to be ejected stably even if the temperature of the print head varies over a wide range.
However, for print heads with heaters of different resistances, the timing at which to switch from the double pulse drive control to the single pulse drive control may vary from head to head.
FIG. 2 is a schematic diagram showing how the drive control method is switched according to the heater rank (dependent on the heater resistance) of the print head and the temperature. In this specification, although the heater rank depends on the heater resistance, it is not determined by the resistance alone. Details of the heater rank will be explained later.
In the diagram, an abscissa represents a head temperature and an ordinate represents a heater rank of the print head. Normally, the print head before printing is set at around 20° C. by a room temperature or by temperature regulation. Depending on the printing operation, the temperature is expected to rise up to around 60° C. The heater rank may vary in a range from maximum to minimum.
In the double pulse drive control, as the heater rank increases, the preheat pulse width narrows early and the drive control needs to be switched to the single pulse drive control at the earliest phase (when the temperature is still low). Conversely, when the heater rank is small, the range in which the ejection volume can be adjusted by the double pulse drive control is wide so that the switching to the single pulse drive control is made at the last phase (when the temperature is high).
When a plurality of print heads or nozzle columns with different heater ranks are mounted on the same printing apparatus, different voltages may be required for different heater ranks. This will make the circuits in the apparatus complex, increasing the overall cost of the printing apparatus. This is not realistic for the ink jet printing apparatus which has a low cost as one of its features.